The European Journal of Mineral Processing and Environmental Protection
Vol.5, No.2, 1303-0868, 2005, pp. 197-201
Technical Note
Influence of additives on physico-chemical properties of
sodium aluminate solution using seed precipitation in the Bayer
process
S. Zhao1,2,*, Y. Yang1, S. Bi1, Y. Xie1
1
Northeastern University, Material and Metallurgy School, Shenyang City,
Liaoning Province, 110004, P.R. China
2
Shenyang Construction University, Shenyang City, Liaoning Province, 110168, P.R. China
Accepted 12 February 2005
ABSTRACT
Various surfactants were added into the sodium aluminate solution in order to investigate the influences
on the precipitation of aluminum hydroxide. The results indicate that proper additives such as C, H2, F, etc.,
can not only improve the precipitation rate, but also enhance the particle size as well as the intensity of the
aluminum hydroxide products. The improvement effects of the surfactants in the Bayer process are ascribed
to the modification of the physical-chemistry properties of the sodium aluminate solutions. Surface tension,
viscosity and conductivity of sodium aluminate solution were studied in detail, and it seems that moderate
decrease of surface tension as well as the decrease of the viscosity is in favor of the production of
aluminum hydroxide with high yield, large particle size and high intensity. © 2005 SDU. All rights reserved.
Keywords: Surfactants; Sodium aluminate; Bayer process
1. INTRODUCTION
One of the major processes in the production of Al(OH)3 is to add the crystal seed, Al2O3 , into the
sodium aluminate solution to precipitate another Al(OH)3 with constant stirring under the temperaturedecreasing condition. In recent years, investigations concerning Bayer process mainly focused on the
factors affecting precipitation of Al(OH)3 from Bayer solution as well as the mechanisms of the process
(Pradhan et al., 2001; Bhattacharya et al., 2002; Paulaime et al., 2003). However, at present, there is still
some products of Al(OH)3 in our country can not meet the requirements of producing sandy Al2O3, because
of the low precipitation rate (Chen, 1986). For that matter, much work has been done by researchers. Recent
reports show that the precipitation process and the quality of the Al(OH)3 can be greatly enhanced by
adding certain amount of surfactants into the sodium aluminate solution (Harrey, 1990; David, 1991; Lester,
1991; Arnwald, 1995; Moody, 1995; Xue et al., 1998). When the surface active substances were added
into the sodium aluminate solution, the physical-chemistry properties would be changed and this will
inevitably influence the quality of Al(OH)3 and its precipitation process.
2. EXPERIMENTAL
2.1. Major apparatus
The major apparatus used are given below: A blade-paddle mixer tank, a Model RPW-09 melting point
comprehensive analyzer, a Model 1054BT electric conductivity apparatus, a Model LB-801 super constant
temperature bath, and capillary viscometer.
* Corresponding author. E-mail: [email protected]
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S. Zhao et al. / The European Journal of Mineral Processing and Environmental Protection
Vol.5, No.2, 1303-0868, 2005, pp. 197-201
2.2 . Methods for measuring
¾ The precipitation ratio η is defined as: η=[1-aα/am]×100%
Where aα denotes the molecular ratio of the primary solution and am stands for the molecular ratio of the
original solution. The measurement of Na2OT, AL2O3 and Na2Ok was performed with the chemical method.
¾ The method for the measurement of the particle size.
The particle size of the Al(OH)3 products was analyzed with a sifter and/or a image analyzer (Wang et
al., 2002). The particle size of the products was expressed with the mass fraction of the particles that was
lower than 45um in diameter.
¾ The method for measurement of intensity
The intensity was performed with a simulated ALCDA attrition index apparatus, with the pressure (N2)
0.40 Mpa and the flow rate of 2.8m3/h lasting 15min. The intensity of Al(OH)3 products was expressed
with the attrition index. The definition is as follows:
AL=[（A-B）/A]×100%
Where A is the mass fraction of the particles that was larger than 45um in diameter before blowing and
B denotes mass fraction of the particles that was larger than 45μm in diameter after the blowing.
2.3 . The experimental materials
¾ The sodium aluminate solution was confected according to the molecular ratio and the concentration
expected with the circulating mother liquor provided by the aluminium factory of Shanxi province.
¾ The surfactants A, G, and C were purchased abroad and the other was produced in our laboratory.
2.4. The experimental conditions
The concentration of alumina is ranging from 160 to 180g/l with the molecular ratio at 1.5, containing
570g/l of the solid seed. The precipitating time is 60h. The temperature was evenly decreased from 75 to
54oC in the whole process.
3. RESULTS AND DISCUSSION
3.1. The enhanced precipitation process of sodium aluminate solution in the presence of additives
In the Bayer process of precipitation, high-intensity, large-size of Al(OH)3 products can be obtained by
adding proper amount of additives. In this study, three kinds of abroad products: A, G and C were employed
and some kinds of surfactants were used which were produced according to the general processes. The
results were shown in Table 1 and 2.
Table 1
The effects of additives on precipitation ratio, particle size as well as intensity of aluminum hydroxide
(addition amount: 100mg/l)
Additive
Precipitation ratio (%)
Number
of
-45μm Attrition index (%)
particle (%)
blank
20.84
24.92
17.21
A
19.24
23.98
13.36
G
21.25
25.84
13.16
C
21.20
24.03
12.15
I
18.74
21.00
16.74
blank
21.06
25.81
17.98
F
21.68
28.02
10.04
H2
22.48
24.94
14.30
H5
17.84
26.92
14.00
H6
16.80
24.32
17.88
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Vol.5, No.2, 1303-0868, 2005, pp. 197-201
Table 2
The effects of additives on precipitation ratio, particle size as well as intensity of aluminum hydroxide
(addition amount: 200mg/l)
Additive
Precipitation ratio (%)
Number
of
-45μm Attrition index (%)
particle (%)
blank
20.96
25.81
17.85
A
21.56
24.11
12.64
G
21.04
24.30
12.68
C
21.88
24.62
12.86
I
19.32
28.24
16.90
blank
20.88
25.56
17.23
F
23.84
24.20
11.80
H2
23.53
26.60
17.98
H5
18.01
26.96
13.77
18.08
24.94
18.87
H6
It can be found from the above two tables that the influences of addition of the additives are differently
according to the kinds they belong to and the amount of the addition. The flowing additives: C, F and H2, can
not only improve the precipitation rate, but also can enhance the particle size and the intensity of the
products. Author had discussed the microcosmic mechanism of additives in relative paper (Zhao et al.,
2003).
The precipitation process was mainly depended on the diffusion and the surface reaction (Xie, 2000).
When the surfactants were added in this process, the physical and chemical properties of the solution such
as the viscosity, surface tension etc. would be apparently changed. Consequently, some surfactants can be
employed, in order to fasten the diffusion rate and enhance formation of the precipitation.
3.2. The influence of the additives on the surface tension of the solution
In order to explore the influence of the additives on the surface tension, various kinds of additives were
employed, keeping the concentration of Al2O3 in the solution from 160 to 180g/l with a molecular ratio of
1.52. A melt comprehensive analyzer was used to measure the surface tension at 75oC. The variations of
the surface tension of sodium aluminate solution with the concentrations of the additives were shown in
Figure1.
surface tension, N/m
0,055
A
0,05
C
0,045
I
0,04
H6
F
0,035
G
0,03
H2
0,025
H5
0,02
0
100
200
300
400
mass concentration, mg/L
Figure 1. Effect of amount of additives on the surface tension
From Figure 1, it can be seen that all of additives used are enable to make the surface tension of the
solution decease greatly, appear a lowest value and then go to a stable state. Of course, different additives
have their own break point of concentration and the decreasing amplitudes are also different. Moreover, it
is not easy to say exactly the relation between the decrease of the surface tension and the enhancement of
the precipitation. Although the lower of the surface tension is favorable to the formation of crystal nucleus,
over decreasing can lead to the high forming rate of crystalline particles, making too much branching
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S. Zhao et al. / The European Journal of Mineral Processing and Environmental Protection
Vol.5, No.2, 1303-0868, 2005, pp. 197-201
crystals on its surface, resulting the intensity decrease. The experiments showed that when the surface
tension of the sodium aluminate solution was controlled in the range from 0.015 to 0.02N/m, the Al (OH)3
products precipitated bear good both intensity and particle size.
3.3. Effect of additives on the viscosity of the sodium aluminate solution
A capipillary viscometer was used to measure the viscosity of the solution in order to investigate the
influence of the additives on the viscosity of the solutions. The operating conditions are given below: the
concentration of Al(OH)3 is 160-170g/l; molecular ratio 1.52 and temperature at 75oC. The whole process
was performed with stirring well.
Figure 2 shows that the viscosity of the sodium aluminate solution can be remarkably changed when the
surfactants were added. Generally, the additives play an important part in the intensity and particle size of
Al(OH)3 because of decreasing the viscosity of sodium aluminate decrease. At the beginning of
precipitation, the viscosity was always below the blank value, making the diffusion speed, which is in
agreement with the precipitation mechanism of the sodium aluminate solution.
350
330
310
A
viscosity, Pa.S
290
C
270
I
250
H6
230
G
210
H2
190
H5
170
150
0
100
200
300
400
mass concentration of additives, mg/L
Figure 2. Effect of the amount of additives on the viscosity
2.4. Effect of additives on the conductivity of solution
In this part, a 1056BT model conductivity apparatus was used to measure the solution conductivity for
studying the effect of the additives on the conductivity of solutions. The original solution was made by 160170g/l of aluminium oxide (molecular ratio 1.52) with various additives at 75C. The results were shown in
Figure 3.
210
A
205
μs/cm
conductivity,
C
200
I
195
H6
F
190
G
H2
185
H5
180
175
0
100
200
300
400
additives mass concentration,mg/L
Figure 3. Effect of amount of additives on the conductivity
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Vol.5, No.2, 1303-0868, 2005, pp. 197-201
The experiments indicated that the five additives: F, H2, H5, H6, and A have little effect on the
conductivity, while G or C can make a little, and only additive I can make it decrease significantly. This can
be understood by the facts that the additive I is a kind of anion surfactant, and when the amount of the
addition is over the CMC (critical micelle concentration), the conductivity became unchanged. While the
degree of ionization of surfactants decreases with the increases of its concentration, as a result, the
conduction of the solution decreases with the increasing of the surfactant concentration (Shen et al., 1997).
For the nonionic surface-active agents such as model F, the addition concentration has little effect on the
solutions conductivity (Zhao, 1996). There seems no relationship between the concentration and the
solution conductivity for the other additives due to the complexity of the system.
4. CONCLUSIONS
Proper surfactants such as C, H2, F, etc., added into sodium aluminate solution can not only improve the
precipitation rate, but also enhance the particle size as well as the intensity of the aluminum hydroxide
product.
The improvement of the precipitation process of aluminum hydroxide can be ascribed to the variations
of the physical-chemistry properties of the sodium aluminate solutions after addition of the surfactants.
Moderate decrease of surface tension of the solution is in favor of the production of aluminum hydroxide
with a better quality. The additives which can lead to the decrease of the viscosity of the solution can
enhance the intensity and particle size of the product. Although there seems no obvious relationship
between the conductivity of the solution and the quality of the product, it can be implied that the best
additives exert their influences by adjusting the crystal growing rate as well as ion diffusion rate in the
solution to optimal values.
ACKNOWLEDGEMENT
This project was financially supported by the National 973 Program (z1999066490-2).
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